Strange behaviour Australia's desert frogs are famously able to store up large amounts of water in their bladder to last them through the drought.

But now researchers from Charles Darwin University in the Northern Territory have found that frog bladders can hold another, even more bizarre surprise.

Dr Chris Tracy and colleagues report in the Royal Society journal Biology Letters that frogs have a unique way of dealing with foreign objects that become lodged in their bodies - they store and even expel them from the bladder.

After finding that frogs implanted with radio transmitters in the field became mysteriously separated from the devices, the researchers implanted small beads into the body cavities of tree frogs and cane toads to see what would happen.

Within nine days, all the tree frogs had expelled the beads completely from the body via their bladder, while the beads in the cane toads had migrated to the bladder and stayed there.

"The bladder of frogs appears to be a unique pathway of expulsion of foreign objects from the body," says Tracy.

Animals such as fish and snakes - and even humans - are known to expel objects out of the body through the skin or the intestine.

But, says Tracy, they were surprised to find that a 2 centimetre long transmitter implanted in the body cavity of an 8 centimetre long frogs could migrate to the bladder.

"When we first started finding transmitters in bladders, we thought wow that's really bad surgery we did. But when we started seeing them over and over again, we realised, 'Hey, this really is something different.'"

Storing from harm's way

The researchers wanted to know how the bead came to be in the bladder, so they dissected implanted cane toads over a number of days to see where it went.

Tracy says tissue grows out from the bladder and wraps around the bead, eventually enveloping it and drawing the bead into the bladder.

"It's pretty remarkable that these amphibians can safely and relatively quickly get rid of potentially dangerous things in the body cavity," he says.

So why would frogs have such a special adaptation?

Tracy speculates that because frogs eat live insects, some of the sharp insect body parts could break off and become lodged in their body.

"The other thing about frogs is their jumping - they aren't very coordinated and they tend to crash land. They have relatively thin skins, so it's easy to imagine a frog landing on something spiny that could poke into their body that way," he says.

"If you have something roaming around inside the body, it's usually a bad thing. It could poke through a vital organ, so it's a good idea to get rid of them."

Tracy says his findings are a cautionary note for other researchers implanting transmitters into animals.

"Finding that your study animal has become separated from your transmitter may not mean it has been eaten by a predator. It might have just expelled the object."